If a pregnant woman is worried lest Sawm should harm her or her embryo, she is allowed to refrain from Sawm, provided that she makes up for the days of Sawm she misses. A pregnant woman falls under the same ruling as a sick person who cannot fast or fears that Sawm may cause them harm. Allah (Exalted be He) says:
...and whoever is ill or on a journey, the same number [of days which one did not observe Saum (fasts) must be made up] from other days.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.